(1) Field of the Invention
The present invention relates to an active optical fiber doped with rare earth element and an optical fiber amplifier, and more particularly to an active optical fiber and an optical fiber amplifier, in which the conversion efficiency of pumping light input into the active optical fiber is increased to improve optical amplification characteristics.
(2) Related Art
In conventional long-distance optical transmission systems, the transmission has been conducted by converting an optical signal into an electric signal, and by adopting an optical regenerating repeater which has functions of retiming, reshaping and regenerating. However, in recent years, optical fiber amplifiers have been put to practical use, so that an optical amplifying and repeating transmission method is now put under consideration in which an optical fiber amplifier is used as a linear repeater. By replacing an optical regenerating repeater with an optical amplifying repeater, the number of parts in the repeater can be reduced to a considerable extent, so that reliability is increased and substantial cost reduction can be expected.
Further, as one choice for realizing an optical transmission system of large-capacity, attention has been directed to a wavelength-division multiplexing (WDM) optical transmission method in which two or more signal lights having different wavelengths are multiplexed and transmitted within a single transmission path.
In a WDM optical amplifying and repeating transmission method which is provided by combining the WDM optical transmission method with the optical amplifying and repeating transmission method, usage of an optical fiber amplifier makes it possible to collectively amplify the two or more signal lights having different wavelengths, thereby enabling economical, of large-capacity and long-distance optical transmission, with a simple construction.
FIG. 22 shows an example of basic construction of a conventional optical fiber amplifier (forward pumping).
In FIG. 22, a pumping light Lp output from a pumping light source 1 is multiplexed, at a multiplexer 2, with a signal light Ls from an input terminal T1, and then input into one end of an active optical fiber 3 depicted by a thick line. This active optical fiber 3 is the one doped with a rare earth element such as Erbium. When an optical fiber amplifier adopting this active optical fiber 3 is unsaturatedly operated, there can be obtained a flat gain wavelength characteristic, thereby realizing transmission of large capacity.
However, when the conventional optical fiber amplifier is unsaturatedly operated, only a part of the power of pumping light which is input into the active optical fiber 3 is used to excite the rare earth element, and the remaining large part of power of pumping light is leaked to the backward side of the active optical fiber. Concretely, approximately 70% of the power of pumping light input into the active optical fiber 3 may pass therethrough, and only about 30% of the power is used to excite the rare earth element. Thus, the conventional optical fiber amplifier has been defective, as having a lower pumping efficiency.
There are also known other conventional optical fiber amplifiers which are contemplated to improve the pumping efficiency, such as disclosed by U.S. Pat. No. 5,138,483. Shown in FIG. 23 is a constitution of this conventional optical fiber amplifier.
In FIG. 23, the optical fiber amplifier, which improved the pumping efficiency, is constituted by modifying the optical fiber amplifier shown in FIG. 22 with an optical reflector 4 which is additionally disposed at a backward side of the active optical fiber 3 (i.e., at an outer side of the end opposite to the input end of pumping light). This reflector 4 reflects the pumping light Lp, and transmits the signal light Ls. The pumping light Lp is reflected by the added reflector 4, to make one reciprocation within the active optical fiber 3, so as to improve the pumping efficiency. Concretely, approximately 50% of the power of input pumping light is used to excite the rare earth element.
However, in the aforementioned conventional optical fiber amplifier which is contemplated to improve the pumping efficiency, just the pumping efficiency may be conversely deteriorated, since the power of the signal light Ls to be output from the optical fiber amplifier is reduced such as due to an insertion loss itself of the added optical reflector 4 and a loss at a part for coupling the active optical fiber 3 to the optical reflector 4.
It is therefore desired to improve the conversion efficiency of the pumping light in the conventional optical fiber amplifier, while enabling high-powerization of output light from the optical fiber amplifier. It is also desired to attain reduction of a noise factor of the optical fiber amplifier, in addition to high-powerization of output light.
In order to high-powerize the optical fiber amplifier, it is required such as: that a sufficient power of pumping light does exist in the part located at an output side of signal light of the active optical fiber; and that an internal loss at an output part of the optical fiber amplifier be reduced.
Further, in order to reduce the noise factor of the optical fiber amplifier, it is required such as: that a sufficient power of pumping light does exist in that part of the active optical fiber which is located at an input side of signal light; and that an internal loss at an input part of the optical fiber amplifier be reduced.
It is also noticed that the power of output light and the noise factor both of the optical fiber amplifier have characteristics different from each other, depending on the wavelength of the pumping light. Namely, with respect to the wavelength of the pumping light, the power of output light of optical fiber amplifier has a characteristic shown in FIG. 24, while the noise factor of the optical fiber amplifier has one shown in FIG. 25. It is therefore seen that a relatively long wavelength of pumping light is preferred for increasing the power of output light, whereas a relatively short wavelength is preferred for reducing the noise factor.